Pregnant and feeling disgusted? You might be having a boy.

Researchers think morning sickness is caused by hormonal changes in the body when women become pregnant. But there’s also a different kind of sickness: feelings of disgust and revulsion to particular food, smells and even experiences.

Polish researchers wanted to examine why some women experience these feelings of disgust. They found that a pregnant woman’s reaction, or “disgust sensitivity,” could predict whether she is carrying a male or female fetus. According to their study, the more disgust a woman feels, the likelier it is she is pregnant with a boy. The findings were published in the journal Physiology & Behavior.

Scientists believe that the feeling of disgust is an evolutionary behavior meant to avert people from coming into contact with pathogens. According to previous studies, male fetuses are more fragile than female fetuses. The Polish researchers theorize women pregnant with male fetuses unconsciously protect them by sniffing out food or other situations that seem suspicious to them.

In the study, 92 women answered a questionnaire measuring their disgust sensitivity at the end of each trimester. Women reported the highest levels of disgust during their first trimester. Women bearing sons reported still feeling relatively high levels of disgust into the second trimester. Women bearing daughters reported feeling less and less disgust as their pregnancies progressed.

In addition to protecting a fragile male fetus, the researchers suggest that women bearing sons must also keep themselves healthy. This may be because male fetuses tend to grow faster and require more nutrients — and the mother must stay healthy to provide all the necessary support for the male fetus.

So men, it may be nice to send an extra nice card on Mother’s Day. One feeling of disgust at a time, they earned it.